Currently, 3GPP is considering development of E-UTRA and E-UTRAN as set out in the technical specification 3GPP TS 36.300 v 8.3.0 (2007-12), incorporated herein by way of reference, and related documents. 3GPP Long Term Evolution (LTE) aims to enhance the Universal Mobile Telecommunications System (UMTS) standard, for example, by improving efficiency and services.
In E-UTRAN, user equipment (UE) communicates with a network node, NodeB (eNB), with data being sent on radio bearers (RBs) over a radio link between them. The eNB interfaces with a Mobile Management Entity (MME) via an interface designated as S1. The E-UTRAN network includes a plurality of eNBs and MMEs. The connection between the UE and the source eNB to which it is attached may be lost, this being known as radio link failure (RLF). It had previously been proposed that a UE experiencing an REF and reappearing in a cell of another eNB would be handled as a transition via an idle state, RRC_IDLE. In this mechanism, when there is RLF in the source eNB, the UE enters IDLE state and selects a new target eNB. When the UE has attached to the new target eNB, and received the system information for the target eNB, the UE may perform the access procedure to establish resources in the target eNB. However, this procedure is time-consuming.
RLF may be particularly likely during fading channel conditions, when handover from the source eNB to a target eNB may be imminent. To avoid going via the IDLE state, it has been suggested that a new eNB could be prepared beforehand to accept the UE by using the handover preparation procedure. The Handover_Request message can be sent to multiple eNBs which are thus able to recognize the UE, each of the eNBs being prepared as handover candidates. Then, the UE is able to carry on with its old context after having chosen the new target eNB during the mobility phase of the RLF.
The paper by Nokia Siemens Networks et al “Radio Link Failure Recovery”, R2-072382, 3GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, discusses RLF recovery by preparing multiple eNBs using the handover procedure. A prepared eNB has the UE context and resources reserved, so by preparing multiple eNBs, the probability is increased that the UE can recover from RLF without needing to go via RRC_IDLE, thus reducing recovery time.
The paper by Nokia and Nokia Siemens Networks “Handover Failure Recovery” R-071717, 3GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, and the paper by Vodafone “Mobility in LTE_ACTIVE state, R2-0723823, GPP TSG-RAN WG2 Meeting #58, incorporated herein by way of reference, also consider the preparation of multiple eNBs during handover to avoid the UE going via the IDLE state.
With reference to FIG. 1, in one LTE network having handover with multiple eNB preparation, a source eNB 1 transmits a Handover_Request to a designated target eNB 2 and to other candidate eNBs 3 and 4, two of which are shown. The Handover_Request causes the designated target and candidate eNBs 2, 3 and 4 to reserve resources, shown at 5, 6 and 7, so that, if the UE attaches to one of the candidate eNBs 3 and 4 during RLF, recovery time is reduced. The eNBs 2, 3 and 4 each send a message Handover_Request_Ack to the source eNB 1 acknowledging the handover request. During RLF the UE appears within a cell associated with the designated target eNB 2, shown at 8. The target eNB 2 sends a Release_Resources message to the source eNB 1 once it has completed the handover procedure. The source eNB 1 then releases resources it has reserved for the UE and sends Free_Resource messages to the other candidate eNBs 3 and 4 to free the resources that they have reserved, shown at 9 and 10.
In another previous proposal, as set out in R2-074179, UE context information is sent to multiple eNBs that are candidates to be the target eNB to which handover eventually occurs, but no reservation of resources is made at these candidate eNBs. This permits a UE to attach to a new eNB within the RLF procedure without going via RRC_IDLE. In a variant, multiple candidate eNBs are sent UE context information and only one candidate eNB, that is the candidate eNB most likely to be the one to which the UE hands over, reserves resources.